1. Field of the Invention
The present invention relates to an optical waveform measurement system and an optical waveform measurement method that measure a waveform of signal light.
2. Description of the Related Art
Optical fiber communication systems operating at a bit rate of 40 Gb/s per channel have been increasingly developed in recent years. The next targets include research on systems operating at a bit rate of 160 Gb/s, and systems operating at a bit rate of 100 Gb/s as the next generation Ethernet (trademark) of 10-Gigabit Ethernet. In this process of research and development, a technology that stably and faithfully observes ultrahigh-speed signal light is required.
FIG. 4 is an example diagram of a configuration of an electrical sampling system of related art. As shown in FIG. 4, the electrical sampling system includes an optical/electrical converter 1, a sampling circuit 2, a waveform display unit 3, a trigger circuit 4, and a sampling pulse generating circuit 5.
After an optical signal is converted into an electric signal by the optical/electrical converter (photo diode: PD) 1, the signal is sampled at the sampling circuit 2 formed by an electronic circuit, and a waveform of the sampled signal is observed by the waveform display unit 3. The trigger circuit 4 outputs a trigger signal to the sampling pulse generating circuit 5, based on a radio frequency (RF) signal synchronized with an optical signal to be measured. The sampling pulse generating circuit 5 generates a sampling pulse at the timing of the trigger signal input from the trigger circuit 4, and outputs the generated sampling pulse to the sampling circuit 2 and the waveform display unit 3.
When a waveform of a signal is observed by the waveform display unit 3, the RF signal synchronized with the optical signal to be measured is required. For example, when signal light of 100 Gb/s is measured, even a band up to the second harmonic component of the signal light is required, due to the limitation of operation speed of electronic circuits, the sampling circuit 2 becomes inoperative.
In related art, there has developed an optical sampling method that breaks a time resolution limit caused by a band limit of an electronic circuit, and realizes an ultrahigh time resolution. FIG. 5 is an example diagram of a configuration of an optical sampling system of the related art. As shown in FIG. 5, the optical sampling system includes a clock extracting unit 10, an optical sampling gate 11, a photo diode (PD) 12, an oscilloscope 13, and a sampling pulse light source 14.
The clock extracting unit 10 extracts a clock signal synchronized with light to be measured, and outputs the extracted clock signal to the sampling pulse light source 14 and the oscilloscope 13. The sampling pulse light source 14 generates sampling pulse light based on the clock signal input from the clock extracting unit 10, and outputs the generated sampling pulse light to the optical sampling gate 11.
The signal light to be measured and the sampling pulse light with a short pulse width are incident on the optical sampling gate 11. The transmittance of the optical sampling gate 11 is controlled by the sampling pulse light. Accordingly, a repetition frequency of light output from the optical sampling gate 11 has the same frequency as an optical sampling pulse. The oscilloscope 13 displays the waveform of the light to be measured, based on the sampling signal converted into the electric signal by the PD 12, and the clock signal input from the clock extracting unit 10.
Therefore, when the sampling light whose repetition frequency is smaller than the frequency of the band limit of the electronic circuit with an ultrashort pulse is prepared, by combining with the optical sampling gate that responds at ultrahigh speed, the sampling of the ultrahigh time resolution can be realized. Accordingly, the light to be measured can be observed by the oscilloscope 13.
For example, an optical sampling of the time resolution of approximately picoseconds is realized, by preparing an optical pulse of a pulse width of picoseconds as an optical sampling pulse, and an optical fiber switch as an optical sampling gate. However, as shown in FIG. 5, the sampling pulse light source should be synchronized with the light to be measured.
Therefore, in general, as shown in FIG. 5, a clock signal synchronized with the light to be measured needs to be extracted, using the clock extracting unit 10. An ultrahigh-speed clock extracting method that extracts a clock signal from a high-speed optical signal at a rate of equal to or more than 100 Gb/s has been proposed.
An apparatus described in O. Kamatani, S. Kawanishi, “Ultrahigh-speed clock recovery with phase lock loop based on four-wave mixing in a traveling-wave laser diode amplifier,” Lightwave Technology, Journal of, Volume: 14 Issue: 8, Aug. 1996, Page(s): 1757-1767 uses an optical phase comparator that includes a semiconductor optical amplifier and a short-pulse light source. Although this apparatus can be advantageously applied for an ultrahigh-speed signal, the size of the apparatus naturally increases. A method shown in C. Boerner, C. Schubert, C. Schmidt, E. Hilliger, V. Marembert, J. Berger, S. Ferber, E. Dietrich, R. Ludwig, B. Schmauss, H. G. Weber, “160 Gbit/s clock recovery with electro-optical PLL using bidirectionally operated electroabsorption modulator as phase comparator,” Electronics Letters, Volume: 39, Issue: 14, 10 Jul. 2003, Pages: 1071-1073 is a method that realizes an ultrahigh-speed optical phase comparator, by controlling an electro-absorption modulator (EAM) by an ultrahigh-speed RF signal of approximately 40 gigahertz, for example. Comparing with the method of “Ultrahigh-speed clock recovery with phase lock loop based on four-wave mixing in a traveling-wave laser diode amplifier”, the size of the apparatus increases because the short-pulse light source is not required. However, because the ultrahigh-speed RF circuit is required in the “160 Gbit/s clock recovery with electro-optical PLL using bidirectionally operated electroabsorption modulator as phase comparator”, the size of the apparatus still remains large.
In a system disclosed in K. Igarashi, K. Katoh, and K. Kikuchi, “High-sensitive optoelectronic clock recovery circuit featuring GVD-independent operation,” ECOC2006, PDP Th4.4.2, Cannes, France, September 2006, an ultrahigh-speed optical phase comparator is formed by generating a frequency chirp using a phase modulation, and taking the chirp component by a light transmissive filter. However, the center wavelength of the light transmissive filter needs to be adjusted, depending on the wavelength of the signal light and the bit rate. Accordingly, an adaptation to the optical sampling that observes ultrahigh-speed signal light of any suitable wavelength is difficult.
To extract a clock signal from the ultrahigh-speed signal light, only an optical phase comparator of ultrahigh speed is required. As the optical sampling system shown in FIG. 5, in the method that separates a part of signal light, and extracts a clock signal from the signal light, the ultrahigh-speed phase comparator is required only to extract the clock signal. Accordingly, the optical sampling system itself becomes very large.
To solve this problem, a system in which an optical sampling gate itself is an ultrahigh-speed optical gate, and uses this gate for an optical phase comparator is disclosed in A. Otani, Y. Tsuda, F. Hirabayashi, and K. Igawa, “Advanced envelope detection method with novel optical PLL for optical sampling trigger,” Electronics Letters, Vol. 41, No. 18, 2005, Pages: 1019-1021. FIG. 6 is an example diagram of an optical sampling system according to the “Advanced envelope detection method with novel optical PLL for optical sampling trigger”. As shown in FIG. 6, the optical sampling system includes an oscillator 20, a voltage controlled oscillator (VCO) 21, a device under test (DUT) 22, an optical sampling gate 23, a display 24, a band bass filter (BPF) 25, a novel optical phase locked loop (NPLL) 26, an operational amplifier 27, a reference voltage controlled oscillator (RVCO) 28, a sampling pulse light source 29, and a PC 30. The NPLL 26 includes an RF signal oscillator (local signal generator) 26a, an FFT unit 26b, a phase comparator (PD) 26c, and an ATT 26d. 
As shown in FIG. 6, for example, signal light of f0=160 Gb/s output from the DUT 22 and sampling pulse light output from the sampling pulse light source 29 are combined, and are incident on the optical sampling gate 23 made of a nonlinear crystal. As is clear in FIG. 6, the optical sampling system does not require an optical clock extracting unit synchronized with the signal light.
The RF signal oscillator 26a is an oscillator that is set within the optical sampling system, and the RF signal (frequency of Δf=f0−Nfs; N is a positive number) output from the RF signal oscillator 26a, and the signal light (frequency of fs, envelope cycle of Δf) output from the optical sampling gate 23 are input into the PD 26c. 
In the PD 26c, phases between the envelope (cycle of Δf) of the signal light output from the optical sampling gate 23 and the RF signal output from the RF signal oscillator 26a are compared, and a signal that has an amplitude proportional to the phase difference is output as an error (error signal). The RVCO 28 obtains the error signal via the operational amplifier 27, and outputs the RF signal based on the obtained error signal to the sampling pulse light source 29, and drives the sampling pulse light source 29.
When the sampling pulse light source 29 outputs pulse light to the optical sampling gate 23, a phase locked loop (PLL) is formed. The optical sampling system sweeps the signal light output from the optical sampling gate 23 by the RF signal taken out from the signal light via the BPF 25, and displays the signal light on the display 24.
With this series of procedures, waveforms can be observed by the optical sampling, without preparing an optical gate specified for the clock extraction, and extracting a clock signal directly from the light to be measured (without enlarging the apparatus).
However, in the above-described related art, waveforms of light to be measured cannot be measured accurately.
In other words, in the technology disclosed in “Advanced envelope detection method with novel optical PLL for optical sampling trigger”, the efficiency of the optical sampling gate 23 is low. Therefore, when a signal waveform with a poor optical signal-to-noise ratio (OSNR) is to be observed, the power of the signal light output from the optical sampling gate 23 becomes low. Accordingly, the sampling light cannot be generated stably, and a faithful waveform cannot be observed.
When the signal light with a large jitter is to be observed, the jitter is also included in the envelope of the signal light output from the optical sampling gate. Accordingly, a jitter component is included in the RF signal that drives the sampling light.